1. Field of the Invention
The present invention relates generally to a communication system employing a high-speed downlink packet access (HSDPA) scheme, and in particular, to an apparatus and method for controlling transmission power upon a change of a best cell during communication in a fast cell selection (FCS) mode.
2. Description of the Related Art
A CDMA (Code Division Multiple Access) communication system typically employs a closed-loop power control technique in order to minimize data loss due to an abrupt change in a channel state (or channel condition). In the closed-loop power control technique, a receiver receives a signal transmitted from a transmitter and detects a power level of the received signal. Thereafter, the receiver determines whether to increase (up) or decrease (down) the detected power level based on the detected power level and then transmits power-up/down commands to the transmitter. If the measured power level of the signal received from the transmitter is lower than a preset level, the receiver transmits a power-up command. However, if the measured power level is higher than or equal to the preset level, the receiver transmits a power-down command to the transmitter.
The power-up/down commands are transmitted using a TPC (Transmission Power Control) bit in a dedicated physical control channel (DPCCH) of a dedicated physical channel (DPCH) established between the transmitter and the receiver. For example, the TPC bit is comprised of a single bit, and has a value of ‘1’ or ‘0’. Herein, the value ‘1’ indicates a power-up command, while the value ‘0’ indicates a power-down command. Thus, upon receiving the TPC bit from the receiver, the transmitter determines its transmission power level based on a value of the received TPC bit. For example, the system may prescribe that the transmitter should increase the transmission power by 1 dB for TPC=1 (indicating a power-up command) and decrease the transmission power by 1 dB for TPC=0 (indicating a power-down command).
Meanwhile, when a user equipment (UE) is located in a soft handover (SHO) region, the UE receives signals from a plurality of active Node Bs belonging to an active set. Thus, there is a demand for a particular power control method in order to properly control power levels of the signals received from the active Node Bs. In the soft handover region, the UE sets up radio links to one or more cells managed by the active Node Bs. The UE receives downlink channel signals from two or more cells, measures power levels of the received downlink channel signals, and transmits a TPC bit having a corresponding bit value to the cells through the radio links. Therefore, when the UE is located in the soft handover region, it creates a TPC bit considering states of the downlink channel signals received from two or more cells.
The existing CDMA communication system employs a basic power control method in which the UE, if it is located in the soft handover region, determines to transmit a power-down command, even though any one of the established radio links has a power level higher than a preset power level. Basically, the UE determines to transmit a power-up command only when the signals received through all of the radio links have power levels lower than the preset power level.
In the case of an uplink, the UE located in the soft handover region may receive TPC commands from two or more cells. The UE then determines whether to increase or decrease the transmission power based on the TPC commands. In the existing CDMA communication system, upon receiving two or more TPC commands, the UE decreases its transmission power if any one of the received TPC commands indicates a power-down command. That is, the UE increases its transmission power only when all of the received TPC commands indicate a power-up command.
Meanwhile, a proposed high-speed downlink packet access (HSDPA) scheme refers to a data transmission scheme for transmitting on a high-speed downlink shared channel (HS-DSCH), which is a downlink data channel for supporting high-speed downlink packet data transmission, and its associated control channels in a future asynchronous mobile communication system, also known as a UMTS (Universal Mobile Telecommunication System) communication system. A typical technique newly introduced to support the HSDPA scheme includes (1) AMC (Adaptive Modulation and Coding), (2) HARQ (Hybrid Automatic Retransmission reQuest), and (3) FCS (Fast Cell Selection).
A brief description of the AMC, the HARQ and the FCS will be given herein below.
In the AMC scheme, if the UE reports the current channel state to a Node B, then the Node B determines a proper modulation mode and coding rate based on the channel state reported by the UE. The HARQ scheme is a retransmission technique for reducing errors on the physical channel. If an error is generated in the initially transmitted data, the HARQ scheme retransmits the same data as the initially transmitted data or transmits additional information for demodulation, thereby increasing demodulation probability of the transmission data. In the FCS scheme, the UE selects a best cell having the best link state among the radio links established by the UE, and reports cell selection information to the Node B of the best cell to receive data from the best cell.
In the HSDPA scheme, the UE receives data from only one Node B, even when it is located in the soft handover region. That is, since the HSDPA scheme supports the FCS scheme, only the Node B managing the best cell having the best reception state among two or more Node Bs with radio links established to the UE, transmits data to the UE. Here, the UE must provide all of the active Node Bs to which the radio links are established, with information on the cell selected by the UE as the best cell. In addition, the UE must transmit ACK (Acknowledgement)/NACK (Negative Acknowledgement) information for the HARQ to the Node B selected as the best cell. The ACK/NACK information is transmitted by the UE exclusively to the best cell, not to the other cells. Therefore, the information received from the respective Node Bs is not combined. The respective Node Bs, especially the Node B of the best cell must receive information from the UE at proper transmission power.
In the soft handover region, the UE accumulates the TPC commands for the current best cell #1 to which the radio link is currently established. For example, if the UE has received the TPC commands of 1,0,0,1,1,1 from the best cell #1, the accumulated transmission power level will be 2 dB. However, when the best cell is changed, i.e., when the current best cell is changed from the best cell #1 to a next (or new) best cell #2, the UE performs transmission power control on the best cell #2 using the accumulated transmission power level of 2 dB for the old (or previous) best cell #1.
When the best cell is changed from the current best cell to a next best cell, the existing CDMA communication system performs transmission power control according to the TPC command for the Node B selected as the next best cell #2 at the point where the best cell is changed, without considering the past TPC command for the Node B selected as the next best cell. For example, the exiting system performs transmission power control without considering the accumulated power level for the best cell #1. Therefore, when the UE transmits data to the next best cell immediately after the point where the best cell is changed, the transmission power level for the next best cell may not be proper. When the UE fails to have a transmission power level proper for the next best cell, the downlink data from the best cell may not be transmitted to the UE. This is because the current transmission power control is based on the soft combining scheme, whereas the HSDPA transmits and receives information of the respective links, which is not subject to the soft combining.
In the conventional transmission power control method based on the soft combining scheme, the UE in the soft handover region decreases its transmission power level upon receiving a power-down command from any one of the Node Bs in the active set. However, the HSDPA system supporting the FCS scheme for performing transmission power control on only the best cell performs transmission power control considering only the next best cell. Meanwhile, there is a great difference between the old best cell and the next best cell in terms of transmission power control tendency for a given duration (period) before the best cell is changed. As a result, there may be a considerable difference between a transmission power level of a signal actually transmitted from the UE and an initial transmission power level of the Node B selected as the next best cell.